This invention relates to a method for the production of a gallium arsenide (GaAs) single crystal.
Of the III-V group compound semiconductors, GaAs exhibits outstanding electron mobility and is finding extensive utility as the crystalline substrate for elements in ultra-high speed integrated circuits and optoelectronic integrated circuits. GaAs is attracting keen attention because (1) when it is of high quality it acquires a high insulating property exceeding 10.sup.7 .OMEGA..cm in resistivity, (2) it can be produced in a grade having minimal intracrystalline defects and enjoying even distribution of such defects, and (3) it can be easily produced in large wafers, for example. As a method capable of producing a GaAs single crystal fulfilling all these requirements, the liquid-encapsulated Czochralski method (LEC method) is receiving increasing attention. This LEC method Czochralski method has a low-pressure version and a high-pressure version. Since the low-pressure LEC method uses as its starting material the GaAs polycrystal formed by the boat growth method, the single crystal suffers inclusion of impurities, predominantly silicon (Si) and requires incorporation of chromium (Cr), a substance capable of imparting a semi-insulating property to the crystal. The addition of Cr in any concentration exceeding 5 ppm by weight entails undesirable segregation of Cr in the crystal. When Cr is added in a concentration of 3 to 5 ppm by weight, the grown crystal suffers a serious defect in thermal conversion. Even when the concentration of Cr added is lowered to the range of 1 to 2 ppm by weight, the grown crystal still suffers a defect in thermal conversion. Another problem is that uniform distribution of the added Cr throughout the grown crystal is no easy task.
By contrast, the high-pressure LEC method which affects synthesis directly on the starting material does not require addition of Cr. Since this method heats and synthesizes Ga and As, the raw materials for crystal, and boron oxide (B.sub.2 O.sub.3), an encapsulant, under high pressure, the melt of raw materials for crystal in the crucible assumes a highly unstable state in the presence of thermal convection. Since this method effects the operation of crystal growth under such an unstable state, the shape of the solid-liquid boundary varies greatly. Owing to the thermal conversion of the melt of raw materials, the grown crystal suffers occurrence of striation and native defect EL.sub.2 level inherent in GaAs. The electric property, therefore, varies greatly within or between grown crystals, and the thermal stability, which has a bearing upon the native defect, is degraded. An integrated circuit of uniform electric property and device property cannot be easily produced with high repeatability using a crystal thus grown as its substrate.
In the production of a Si single crystal, it is a widely used practice to improve the quality of the grown crystal by applying a magnetic field to the crystal under growth. However, it has been considered that application of a magnetic field to the melt of raw materials for growing GaAs single crystals under high pressure would cause the melt to be shaken. Further, since a GaAs single crystal manufacturing apparatus has a more complicated construction than that of a Si single crystal manufacturing apparatus and it is difficult to apply a magnetic field to a crucible containing therein a melt of raw materials, application of a magnetic field has not been carried out as likely as not.